This invention relates to ferrous metal alloys with surfaces protected by Si compositions and, more particularly, to Fe-Cr alloys with protective coatings containing Si and/or Si-O for use in oxygen/sulfur mixed-gas environments.
Ferrous metal alloys and particularly Fe-Cr or Fe-Cr-Ni alloys may be identified as structural alloys and have use in the fabrication of coal gasifiers, coal combustion units, and other equipment designed to operate at temperatures in the order of 500.degree.-1000.degree. C. Upon exposure to aggressive gaseous environments at elevated temperatures, these alloys depend on their ability to form and maintain protective surface oxide scales to minimize the rate of corrosive degradation. However, in oxygen-sulfur mixed-gas environments, typical of those encountered in coal gasification and combustion atmospheres, it has been determined that most of the structural alloys will tend to have difficulty developing an oxide scale and that in the absence of protective oxide layers, the rates of alloy degradation, due to sulfur attack, will be prohibitively high for practical applications.
Also, even if a structural alloy is capable of developing a protective oxide scale after short-term exposure to mixed-gas environments, the life expectation for the alloy is strongly dependent on whether the alloy exhibits "breakaway" or "accelerated" corrosion. This type of corrosion may be a particular problem under significant changes in temperature. Most of the structural alloys, in general, exhibit breakaway corrosion, especially in oxygen/sulfur mixed-gas atmospheres with the exposure time at which it occurs being dependent on temperature, gas chemistry, alloy composition and scale microstructure. During temperature cycling conditions, breakdown of scales can occur due to the difference in thermal expansion properties between scales and alloy substrates due to growth stresses that are generated during oxidation. In order to minimize corrosion rates for an alloy under exposure to sulfur environments and therefore to minimize breakaway corrosion, it is important to minimize the outward transport of cations and inward transport of sulfur with respect to the surface alloy. One of the techniques for lowering the corrosion rates for a structural alloy is to develop a barrier layer on the surface of the material to be protected in the service environment.
As reported in K. Natesan, "Corrosion and Mechanical Behavior of Materials for Coal Gasification Applications", Argonne National Laboratory Report ANL-80-5, May, 1980, barrier layers as coatings of Al or Cr have been formed or applied on structural alloys with varying results. Several have been porous and limited the desired protection provided to the underlying alloy. In general, the coatings have been 10 mils or more in thickness and have separated or broken away from the underlying alloy when the coated alloys have been subjected to temperature cycling conditions.
Accordingly, one object of this invention is a structural alloy with a surface protected from oxygen/sulfur mixed-gas environments. Another object of the invention is a structural alloy with a nonporous, protective surface coating. Yet another object of the invention is a structural alloy with a protective surface coating resistant to separation from the underlying alloy. A further object of the invention is a structural alloy with a protective surface coating forming a barrier to sulfur attack on the underlying metals.